Definition
DPI vs PPI
PPI is the pixel density of an image file; DPI is the dot density a printer lays down. Print work asks for 300 PPI at final printed size — an image's pixel count only means something once you fix how large it will print.
The number that matters is pixels divided by printed inches. A 1200-pixel-wide image is 300 PPI across four inches and 150 PPI across eight, so a file can be simultaneously fine and unusable depending on placement. Nothing is gained by resampling upward: the pixels are not recovered, only interpolated, and the softness survives. Line art is the exception, wanting far higher densities because a stepped edge is more visible than a soft photograph.
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